Electrical And Magnetic Properties And Doping Effect In The Perovskite Manganites

The perovskite manganites possess great applying potential and relate to many basic physical mechanism because of colossal magnetoresistance (CMR) effect. After that the CMR was found in the ABO3 type manganites, the similar effect was observed in the Sr3Ti2O7 type manganites. But compared with the former, there are less reports about the latter. On the one hand, getting the manganite sample with Sr3Ti2O7 type structure and high quality is more difficult than getting the sample of the ABO3 type manganite, on the other hand, the physical mechanism in the Sr3Ti2O7 type manganite is more complicated. This thesis emphasized on the study of the condition in which the Sr3Ti2O7 type manganite can be formed and its electrical transport, magnetism, magnetoresistance, doping effect etc. The major study and some of the conclusions are listed below: The advances in the study of MR effect and MR material have been summarized. More attentions have been paid to the crystal structure, electric and magnetic properties of the ABO3 and Sr3Ti2O7 type manganites. The two kinds of manganites were compared generally and the basic thinking of subject selection and investigation content was put forward. In order to investigate the condition in which the Sr3Ti2O7 type structure can be formed, the condition in which the ABO3 type structure can be formed was investigated. For the solid state reaction method, the high sintering temperature(~ 1400oC) is required to get the single phased polycrystalline sample. But for the sol-gel method, the single phased polycrystalline sample can be formed in a wide sintering temperature (600 –1450 oC). The X-ray diffraction and microstructure show that in the selected sintering temperature range, though the sintering temperature has no obvious effect on the forming of the ABO3 type phase, the sintering temperature strongly affects the grain size, with decreasing the sintering temperature, the grain size becoming small, the grain boundaries increasing, it directly affects the electrical transport properties. Based on the preparation of the ABO3 type manganites, the condition of phase forming for the Sr3Ti2O7 type manganites was investigated. For the sample with nominal composition La4/3Ca5/3Mn2O7, although many samples were prepared, the Sr3Ti2O7 type structure were not formed. Combining the structure analysis, We consider that the sample prepared according to the nominal composition La4/3Ca5/3Mn2O7 is easy to form the mixture of La2/3Ca1/3MnO and CaO. For the sample with nominal composition La4/3Sr5/3Mn2O7, the Sr3Ti2O7 type structure can be formed, but it is only can be formed in proper sintering temperature and sintering time. If the sintering temperature is too high and the sintering time is too long, the ABO3 type phases will form on the surface. The penetration depth of the ABO3 type phases into the inside of sample is deep for high sintering temperature and long sintering time. If the sintering temperature is too low, the sample is mainly composed of the ABO3 and K2NiF4 type phases. On the basis of preparation a lot of samples, to get the Sr3Ti2O7 type manganites with high quality, the best sintering temperature and sintering time is 1400oC and 12h. We have successfully prepared the polycrystalline La4/3Sr5/3Mn2O7 sample with high quality and Sr3Ti2O7 type structure. The electrical transport, magnetism and magnetoresistance properties of the two kinds of manganites were investigated. For La2/3Ca1/3MnO3, with the grain size decreasing, the insulator-metal transition temperature decreasing. Although the paramagnetism-ferromagnetism transition temperature mainly maintain a constant, the transition temperature range becomes wide, the saturation magnetization decreases, and the intrinsic magnetoresistance near the transition temperature related to the Mn spins orientation decreases, the extrinsic magnetoresistance below the transition temperature related the spin polarized tunneling increases. For the samples with nominal composition La4/3Ca5/3Mn2O7, the electrical transport, magnetism and magnetoresistance are all similar to those of La2/3Ca1/3MnO3, and the transition temperature is close to that of La2/3Ca1/3MnO3. The similarity comes from that the La4/3Ca5/3Mn2O7 sample actually is composed of La2/3Ca1/3MnO3 and CaO. The electrical transport, magnetism andmagnetoresistance of La2/3Sr1/3MnO3 are also similar to those of La2/3Ca1/3MnO3. But in sharp contrast to La2/3Sr1/3MnO3, the transition temperature of La4/3Sr5/3Mn2O7 with Sr3Ti2O7 type structure is much lower than that of La2/3Sr1/3MnO3, La4/3Sr5/3Mn2O7 not only shows large MR near the transition temperature, but also shows large constant MR below the transition temperature, the large MR below transition temperature comes from the electrons tunneling between the adjacent (MnO2)2 layer. For the sample with ABO3 type phase on its surface, the electrical transport properties of the samples with the surface layer and without the surface layer were investigated intentionally. For the sample with the surface layer, the insulator-metal transition temperature is above 200K and the resistivity is low. But for the sample without the surface layer, the insulator-metal transition temperature is ~ 120K and the resistivity is high. The difference in the properties of La4/3Sr5/3Mn2O7 and La2/3Sr1/3MnO3 is induced by the difference in the structure. Based on that the La4/3Sr5/3Mn2O7 with the Sr3Ti2O7 type structure is successfully prepared, doping at La, Sr and Mn sites, the effect of doping on the structure and electrical transport properties were investigated. Lightly doping of Ca or Ba can substantially improve the MR effect, but if the content of Ca is larger than 50% or the content of Ba is larger than 20%, the Sr3Ti2O7 type structure is instable and it will phase separate into ABO3 type manganites. Doping at La and Mn sites usually leads to the insulator-metal transition temperature decrease, but Cr and Mn doping at Mn sites causes the insulator-metal transition temperature increase. For the doping effect in the light Ca, Ba, Nd, Bi and Y doped samples, based on the stability of the orbit of eg electrons, the reasonable explanation is given. For the change of structure in the heavy Ca and Ba doped samples, it can be explained by the mismatch effect inducing the structure phase separation. For the complicated doping effect in Mn sites, the mechanism needs to be investigated further. We also prepared the La2/3(Ca1-xMgx)1/3MnO3 samples and investigated the effect of Mg doping on the structure, electrical and magnetic properties. The results show that Mg ionsare not existed in the lattice of La2/3Ca1/3MnO3, but the doping of Mg greatly improves the MR effect near the insulator-metal transition temperature.